When would Sisko have had a chance to command a Corps of Engineers unit?

When we were introduced to him, he was already a lieutenant commander, and executive officer of a starship. We learned in "Home Front" that he was also previously an executive officer on a different starship (presumably, but not definitely, at lieutenant commander). We know his whole career after that. So, where was he before the Okinawa?

We know that Sisko, as an ensign, was also involved in whatever Curzon Dax was doing, presumably something involving diplomacy and somehow involving the starship Livingston. Perhaps providing security as a personal bodyguard? Why else would a young officer be involved with a fiesty diplomat? Was he in some kind of mentorship program for command-track junior officers?

We don't really know what Sisko was doing when he was a lieutenant, but had to have been engineering, and it is probable that he was an engineering officer on the Okinawa as a lieutenant, maybe he was the chief engineer. Then Leyton made him his XO, and the rest is history....